1. Field of the Invention
This invention relates to a method for producing essentially haze-free isophthalic acid- and polymethylolalkanoic acid-containing resins. More particularly, this invention relates to selectively catalyzing the esterification of the carboxyl moiety of isophthalic acid in preference to the carboxyl moiety of polymethylolalkanoic acid to produce essentially haze-free resins.
2. Description of Related Art
Polyol-polybasic acid, thermosetting (polyester) resin compositions are used extensively in preparation of paints, enamels, and other protective coatings. Both water-soluble and organic-soluble resins have been prepared, thus yielding both water-based and solvent-based protective coating compositions. Such compositions are commercially valued because resin compositions are relatively low cost, are easily applied, and yield high-gloss protective coatings which are resistant to attack by weather, adhere well to many substrates, and are tough and flexible.
Alkyd resins comprise a polyester resin modified with an oil, such as soybean oil, linseed oil, and the like, or a refined fatty acid. Alkyds are relatively economical vehicles for solvent-based paint products. The oil or fatty acid component is incorporated into the resin molecule to modify the properties of the resin, for example, to make the alkyd resin suitable for use in protective coating products which air dry at ambient temperature. Metal catalysts may be incorporated into the coating product to catalyze the crosslinking of the unsaturated oil or fatty acid side chains. Those polyester resins containing no side-chain unsaturation are usually cured by baking.
Both polyester and alkyd resins can be made water soluble, typically by incorporation of water-solubilization moieties such as free acid groups. The acid group is solubilized by neutralization, as described herein.
Water-soluble alkyd resins are preferred because of the enactment of regulations which restrict the quantity of organic solvent that can be introduced into the environment per volume of coating product and because of the newly-heightened awareness of the toxicity and flammability of organic solvents. Further, not only is the cost of water typically lower than the cost of suitable organic solvents, but also water-borne coatings are easily and less expensively applied. Where organic solvents are required for cleaning equipment and, for example, for removal of coating accidentally applied to a surface, water-borne material requires only water for these purposes.
Typically, an alkyd resin or polyester resin prepared from only polyols and polybasic acid constituents is not water-soluble. Therefore, water soluble moieties must be incorporated into the resin. A constituent having a functional group, such as a carboxyl group, which can be made water soluble, is incorporated into the resin in a first step. Thereafter, the functional group is reacted to convert it to a water-soluble moiety and make the resin water soluble.
Various constituents, such as trimellitic anhydride, can conveniently be incorporated into an alkyd or polyester resin through esterification of the anhydride group with hydroxyl groups on resin molecules. The third carboxyl-functional group remains unreacted, and, upon reaction with an amine or ammonium hydroxide, increases the water solubility of the resin.
Polymethylolalkanoic acids are especially suitable to provide water solubility. When such an acid is incorporated into resin by esterification of methylol moieties, the carboyl moiety remains unreacted, and can be reacted to make it water soluble, as described above. However, a polybasic coupling acid must be utilized to incorporate polymethylolalkanoic acid in this manner. One carboxyl group on the coupling acid esterifies a methylol moiety on the polymethylol-alkanoic acid, while a second carboxyl group esterifies a hydroxyl group on the resin molecule.
Aromatic dibasic acids such as phthalic acid or anhydride and isophthalic acid are often used as the polybasic component in alkyd resin formulations. In water soluble alkyd resins used in protective coatings, isophthalic acid is preferred over phthalic acid or anhydride. In the case of water soluble resins, there exists the possibility that the ester linkages will hydrolyze. Isophthalic acid is preferred because it is believed to improve the hydrolytic stability of water-borne ester resins produced therefrom.
Various methods of producing polyesters as precursors to polyurethanes, polyureas, and other alkyd resins, are known in the art. For example, U.S. Pat. No. 3,658,939 discloses use of a carboxyl containing organic diol, such as dimethylolpropionic acid, to extend the chain length of a polyurethane. Other components of these polyurethanes are polyester polyols prepared by the esterification of of dicarboxylic acid or anhydride using a linear alpha,omega-diol in the presence of a catalyst such as dibutyltin oxide or stannous oxalate with lithium acetate. These polyester polyols are used in combination with isocyanates to form polyurethanes.
A method for preparing aromatic polyesters is disclosed in U.S. Pat. No. 4,093,595. According to the method, dihydric phenol and aromatic dicarboxylic acid are directly polymerized in the presence of an esterification catalyst selected from the group consisting of antimony, tin, and titanium compounds. In U.S. Pat. No. 4,166,149, polyesters which are the reaction products of dihydric alcohols with dibasic carboxylic acids are combined with diisocyanate to yield a urethane prepolymer. Then, diamine chain extenders are introduced to increase the molecular weight of the resulting polyurethane polymers.
U.S. Pat. No. 4,356,285 is directed to thermosetting resins and to methods of making them. One of the components of the resin is the reaction product of a carboxylic acid and an alcohol. Formation of this reaction product can be catalyzed by organometallic, metal oxide, metal acetate, or metal alkoxide catalysts. The reaction product is one component in the thermosetting resin.
A method for producing essentially linear polyesters having low carboxyl content is disclosed in U.S. Pat. No. 3,787,370. Although the patent is directed to the use of orthocarbonate catalysts for the esterification, the specification describes a known reaction of aliphatic dicarboxylic acid or hydroxycarboxylic acid with glycol, optionally in the presence of known esterification catalysts, to produce a glycol ester. This glycol ester subsequently is polycondensed, liberating a glycol molecule in the process. The polycondensation proceeds at an exceedingly slow rate, even when antimony-, titanium-, or germanium-containing catalysts, or catalysts such as zinc acetate, manganese acetate, or lead oxide, are used.
None of the above-described patents teaches a method whereby polymethylolalkanoic acids are utilized to improve the water-solubility of alkyd resin. However, various two-step processes for including water-solubilizing agents in polyesters which are part of alkyd resins are known in the art. One such process is described in U.S. Pat. No. 3,345,339. In a first step, isophthalic acid, terephthalic acid, or a mixture thereof, is esterified with a polyol in the presence of inorganic tin salt esterification catalyst. The second step adds an alpha, beta-unsaturated polycarboxylic acid to the resin. In an example in the patent, a resin comprising the polyethylene glycol ester of terephthalic acid is prepared, then is reacted in a second stage to incorporate maleic anhydride into the structure and produce an acceptable haze-free product. Simultaneous reaction of these components in one stage, however, yields an unacceptable product that was not haze-free.
Other known two-stage processes use butyl stannoic acid, tetrabutyl titanate, stannous oxalate, and dibutyltin oxide as esterification catalysts. Typically, in the first stage, an alkyd resin is prepared, and a solubilizing agent is incorporated in a second stage. Trimellitic anhydride is commonly incorporated in such a process; dimethylolpropionic acid also can be utilized. However, isophthalic acid cannot be used as the coupling acid for dimethylolpropionic acid because the carboxyl moiety of dimethylolpropionic acid tends to react faster than isophthalic acid. Thus, isophthalic acid does not completely react and makes the resin hazy.
A method for manufacturing water-soluble alkyd resins is disclosed in U.S. Pat. No. 3,345,313. Therein, a polyhydric alcohol, a polycarboxylic acid or anhydride, and a polymethylolalkanoic acid having between about five and seven carbon atoms are reacted together without catalyst to produce a condensation polymer containing free carboxyl moieties. The patent notes that these carboxyl functionalities can be modified to increase the solubility of the resin. The polymethylolalkanoic acid is incorporated into the polymer chain by esterifying the acid groups on the polycarboxylic acid with the hydroxyl moieties on the polymethylolalkanoic acid. Because the carboxy moiety on the polymethylolalkanoic acid is sterically hindered, uncatalyzed esterification of this moiety, if it occurs at all, proceed exceedingly slowly. Thus, the carboxyl moiety on the hindered alkanoic acid does not react under mild conditions taught in the patent, i.e., no catalyst and reaction times of up to 30 hours.
These methods are unsatisfactory, as they do not provide a method for producing a water-soluble, essentially haze-free resin containing isophthalic acid and polymethylolalkanoic acid wherein isophthalic acid is essentially completely reacted in the presence of polymethylolalkanoic acid. Because isophthalic acid is not soluble in resins, incomplete esterification thereof can lead to its crystallization in the resin product. Not only is the appearance of a hazy resin commercially and cosmetically unacceptable, but also isophthalic acid crystals will probably generate film defects in a protective coating. Although a minor amount of haze can be tolerated, excessive resin haze typically is removed by filtration to make the resin commercially acceptable. If the crystals do not appear until the product has been shipped, or worse, until the product has been used and the protective coating has failed, the manufacturer of the resin will be faced with the significant costs of rectifying the situation, and may be faced with the loss of a customer.
Two-step processes, such as the one disclosed in U.S. Pat. No. 3,345,339, are unsatisfactory for several reasons. Addition of the polymethylolalkanoic acid in the second stage of the reaction requires the addition of solids to a heated reaction vessel. Such an operation is hazardous and typically is very difficult. Further, the viscosity of the reactant mixture may increase rapidly, making agitation difficult, or a gel may form, thus yielding an unacceptable product.
The range of products obtained from such two-step processes typically is limited. For example, the product of U.S. Pat. No. 3,345,339 is an unsaturated polyester not suitable for use in a protective coating product. Inclusion of polyhydroxymonocarboxylic acid into a polyester without esterifying the carboxyl moiety of the polyhydroxymonocarboxylic acid requires the use of coupling acids. Where water-solubility is desired without regard to the drying characteristic of the resin, acids such as adipic acid and azeleic acid are suitable coupling agents. However, because these acids are detrimental to the air-drying characteristic of resins thus made, phthalic anhydride typically is used in air-drying alkyd resin compositions. Phthalic anhydride often is perceived as commercially unacceptable, and the demand for resin containing only isophthalic acid is high. Although isophthalic acid would be a suitable coupling acid, its use in known one-step methods is precluded by the above-described haze formation problem.
It is an object of this invention to provide a method for producing essentially haze-free isophthalic acid- and polymethylolalkanoic acid-containing resins.
It is another object of this invention to provide a method for producing essentially haze-free isophthalic acid- and polymethyl olalkanoic acid-containing resins in one stage or reaction step.
It is yet another object of this invention to provide a method for selectively esterifying the carboxyl moiety of isophthalic acid in preference to the carboxyl moiety of polymethylolalkanoic acid to produce essentially haze-free water-soluble resins.
It is still another object of this invention to provide alkyd and polyester resins comprising essentially haze-free isophthalic acid- and polymethylolmonocarboxylic acid-containing resins.
It is a further object of this invention to provide a method for producing a polyester or alkyd resin comprising polymethylolalkanoic acid as a water-solubilizing moiety incorporated into the ester by esterification of a methylol moiety thereon with isophthalic acid.
It is still a further object of this invention to provide protective coating products comprising these polyester and alkyd resins, and objects protected with the coatings.